High efficiency oxide coated cathode and method of manufacture



E. F. LOWRY July 31, 1934.

HIGH EFFICIENCY OXIDE COATED CATHODE AND METHOD OF MANUFACTURE Ei ledMay 20., 1932 mm m w #WI'W INVENTOR ATTORNY Patented July 31, 1934UNITED STATES PATENT OFFICE HIGH EFFICIENCY OXIDE COATED CATH- ODE ANDMETHOD OF MANUFACTURE Pennsylvania Application May 20, i932, Serial No.612,505

12 Claims.

This invention relates to cathodes and particularly to cathodes for usein connection with gas fllled tubes such as frequently used forrectifiers.

It is an object of this invention to obtain the desired quantity ofelectron emission with a smaller expenditure of heating energy than hasheretofore been required.

It is a further object of my invention to prom vide a cathode comprisinga heating member and a radiation shield both of which shall participatein the electron emission.

It is a further object of my invention to produce a ribbon of cathodematerial capable of being wound edgewise.

It is a further object of my invention to provide a cathode structure oflarge electron-emission surface and relatively small effective radiationsurface.

It is a further object of my invention to cause the heater of anindirectly heated cathode to function also as 9. directly heatedcathode.

It is a further object of my invention to produce a combination ofheater acting as a directly a heated cathode and radiation shield actingas an indirectly heated cathode in which the effective radiation surfaceof the heater shall be of nearly the same area as the electron emissivesurface of the shield. Because the effective radiation surface of theheater is nearly equal to the adjacent surface of the shield and bothtake part in the electron emission I secure a more nearly uniformtemperature throughout the whole cathode. a It is a further object of myinvention to provide a cathode structure which shall avoid localoverheating by the space current.

It is a further object of my invention to provide ample room for passageof electrons from the cathode to the anode while in effect completelysurrounding the cathode with a radiation shield.

It is a further object of my invention to manufacture a helix from astrip, the faces of the strip being edgewise toward the axis of thehelix, from a material which is not readily stretched.

Other objects of my invention and details of the construction will beapparent from the following description and the accompanying drawing towhich reference is made.

In the drawing, Figure l is a longitudinal section of the cathode.

Fig. 2 is a view in section taken along the line 11-11 of Fig. 1.

Fig. 3 is a View partly in section and partly in elevation of a tube inwhich the cathode structure is used.

Fig. 4 is a view in front elevation and Fig. 5 is a view in sideelevation of apparatus used in the manufacture of the cathode.

Fig. 6 is a view in side elevation of another apparatus used in anotherstep of the manufacture.

Referring to the drawing and in particular to Figs. 1 and 2 thereof, thecathode comprises a strip 1 of nickel, ferroAccbalt-titanium alloy,tungsten or other metal suitable for cathodes, in combination with ashield 2. The strip is in the form of a helix. The width of the stripbeing nearly normal to the axis of the helix. The strip is corrugated,the corrugations 3 extending transversely of the strip and preferablymaking a small angle with the radius of the helix. The length of any onecorrugation thus extends across the strip but preferably is not normalto the edges thereof. Instead it makes a small angle, such as 15, 75with such normal.

These corrugations are deeper at the inner edge of the strip and areflatter at the outer edge. They may even completely disappear at theouter edge so that the extreme outer margin of the so strip isuncorrugated.

At the upper end of the helix the strip is bent upwardly, as shown at 4,for connection to a lead 5 and at the lower end it is similarly bent asshown at 6 for connection to a lead '7.

The shield 2 is the inner one of a plurality of concentric cylinders 8,concentric with the helix. The helical portion of the strip 1 and theinner surface of the cylinder 2 adjacent thereto are coated with anelectron emissive material, such as the oxides of barium and strontium.The coating upon the inner faceof the cylinder 2 extends the length ofthe helix as indicated at 11, but the cylinder itself extends beyond thehelix in both directions and is rigidly connected to the other cylinders8 by spacing and supporting devices 12 near the top and the bottom ofthe shield.

At the top of the shield structure a plurality 01' supports 13 space theend shield 15 from the edges of the cylindrical shields 8, therebyleaving a y space between said upper edge and the end shield 15 which isunobstructed except by the supports 13. The edge of the shield 15 ispreferably turned downwardly forming a flange 16 and the supports 13 aresecured to the outer surface of 1 5 the outermost cylinder 8 and to theinner surface of the flange 16.

At the bottom of the cylinders 8 they are closed by a bottom shield 20which is fitted to the outermost cylinder 8 by an up-tumed flange 21. An1.10

opening 22 is provided in the bottom shields 20 for the passage of thelead 7.

The lead 5 connects the upper end 4 of the strip to the cylinder 2. Thisconnection may extend to the other cylinders either through the lead 5or through one or more of the spacing connections 12. The upper end ofthe strip is thus connected to the bottom member 20 of the shield.

A lead 24 is connected to the bottom member 20 and the leads 7 and 24extend through the press to the outside of the tube 25 as shown in Fig.3. The anode 26 is connected through the usual seal to an externalterminal 27. The circuit in which the tube 25 is used includes aconnection from the terminal 27 to the terminal associated with the lead24.

The power for heating the cathode is delivered to the leads 7 and 24.The correct phase relation should be maintained in making thisconnection. At the time that the anode terminal 27 is positive, that is,while the tube is conductive, the heating current should be in such adirection that the lead 7 is positive with respect to lead 24. This hasbeen noted upon Fig. 3 by applying the sign to terminal 27 and the lead7 and the sign to the lead 24. These signs thus stand for the relationbetween the instantaneous potentials during that half-cycle throughoutwhich the tube is conducting.

In order to manufacture the helical strip 1, a strip of metal suitablefor use in a cathode is first passed between a pair of gears shown inFigs. 4 and 5. The gears 30 and 31 are provided with intermeshing teethwhich make an angle, preferably about 15, with the elements of the pitchcylinder of each gear. The upper gear 31 is mounted adjustably asindicated by the blocks 32 and the set screw 33, while the lower gear 30is mounted in fixed blocks 34.

The depth of the corrugations produced in the strip by means of thegears is regulated by the adjustment of the gear 31. When the strip hasbeen passed through these gears it has a series of uniform corrugationsextending obliquely across it at an angle of some 15 with the width ofthe strip.

In the appaartus shown in Fig. 6 a squarethreaded screw 37 is mounted inany suitable support and driven manually or in any convenient manner.The outer end of the screw thread affords a shoulder over which an endof the strip is hooked as indicated at 38. Rotating the screw thencauses the strip to be wound upon the screw, the strip entering betweenadjacent walls of the thread.

During this operation the ends of the corrugations near the outer edgeof the strip which are stretched, and thus the corrugations areflattened at their outer ends. The width of the space between adjacentturns of the thread of the screw 37 is sufficient to accommodate thefull height of the corrugations. At the inner edges of the strip,therefore, the corrugations are of full depth and are not crushedtogether orflattened down by any pinching action of the screw.

After it is wound on the screw, the distance along the strip betweenadjacent corrugations is smaller at the inner edge. The corrugationsthus become steeper and deeper at their inner ends and at their outerends are flattened and may even be completely flat. This is because thewinding has brought the corrugations nearer at their inner ends andseparated them at their outer ends.

When the winding of the strip upon the screw 37 is completed, the woundstrip is removed from the screw by rotating the screw in the oppositedirection. The strip thus formed is self-supporting. It possessesconsiderable rigidity against compression by pushing the ends of thehelix toward one another or extension by pulling the ends away from eachother. The wound strip is also rigid enough against bending in a planeparallel to the axis to make it capable of supporting itself in ahorizontal position. In some applications of my invention I find itconvenient to use a device with the axis of the strip horizontal.

The stiffness is more than that corresponding to the natural stiffnessof the material. The corrugations and their position at a small angle tothe radius add stiffness to the structure.

After the helix has been made, all of it except the tabs 4 and 6 iscoated preferably with a mixture of carbonates of barium and strontium,which coating changes during the usual treatment of the tube to amixture of the oxides of said metals. Before this is done, the innersurface of the cylinder 2 is coated with an electron-emissive materiallike that on the strip, the coating extending from a location adjacentthe upper end of the helix to one adjacent the lower end. An uncoatedportion of the inner surface on the cylinder 2 extends beyond the helixin each direction.

The leads 7 and 5 constitute anchorages by which the ends of the helixare held stationary. They are welded or otherwise connected to the tabs4 and 6. v

The material for the shields 8 must have good reflecting properties andthe innermost shield 2 must not so react with the material with which itis coated, even at high temperature as to diminish the thermionicactivity. Nickel is found to answer these requirements well. The shieldsare secured in place by mounting them upon the bottom shield 20 and thecover shield 15 is secured to the supports 13. The cathode is then readyto be mounted in the tube 25 and the tube is completed by the usualheating, pumping, etc. A small quantity of mercury vapor or otherionizable gas such as neon, argon, etc. is left in the tube when thepumping is finished. The device in operation is connected into anysuitable or usual rectifier circuit. The heating current is preferablyalternating and of the same frequency as the current to be rectified. Itis introduced into the cathode by connections to the terminals of theleads 7 and 24. The anode is connected through the terminal 27 and theusual circuit to the source of current to be rectified. The lead 24 actsas the other connection to the rectifier circuit. The phaserelationships are so chosen that at the time the connection to theanode-terminal 27 is positive the connection to the lead 7 is alsopositive.

While the device is conductive, that is, while the anode is positive,the electrons which constitute the space current emerge from the innersurface of cylinder 2 and from the strip 1 and move to the anode. Thespace current circuit is over the lead 24 and the electrons which emergefrom the cathode are returned to it over this lead. The electronsemitted from the cylinder 2 constitute a portion of the space currentand the strip 1 is thus relieved from conducting that portion of thespace current.

Since the area of the coated surface of the cylinder 2 is of the sameorder of magnitude as the area of the coated strip, the space current,

carried by the strip is reduced by an important amount, namely theamount corresponding to the emission of the coated surface of thecylinder 2. The heating effect of the space current in the strip 2 isthus reduced to an amount such that undesirable overheating is unlikelyeven at the end 4 where the strip carries most current.

The strip 1 and the surface 11 emit electrons, the strip having beenheated by the current flowing therein and the cylinder 2 by radiationfrom the strip 1. Although the heating current flows through thecylinder 2, the cross section of said cylinder is sufficient to carrythis current without much heating. The shield therefore, is not heatedto any substantial amount by the heating current.

The heat radiated by the strip 1 is absorbed by the coated portion 11 ofthe cylinder 2 and reradiated into the space containing the strip 1.comparatively little radiation occurs from the outer surface of thecylinder 2 because it is a bright surface. It tends, therefore, toreflect any heat reaching it from the strip 1 back toward said strip.Similar reflections occur at each surface of each of the shields 8,except the inner surface of the shield 2. The combined effect of all theshields is to minimize loss of heat by radiation from the cathode.

It will be observed that electron emission occurs throughout a largesurface including the corrugated surface of the strip 1 and the coatedsurface of the shield 2. Because the tube is gasfilled, the narrowspacer between corrugations or those between whorles of the helix emitelectrons substantially as effectively as an unobstructed surface.

The same is not true of the heat radiated from such closely adjacentsurfaces. The surface of the strip 1 is for the most part at a largeangle to the direction in which radiation away from the helix musttravel. The space occupied by the helix behaves very much like a blackbody", the radiation therefrom being practically the same as theradiation from the cylinder which would be the envelope of the helix.This isat least approximately the same area as the inner surface of theshield 2 because the diameter of this shield exceeds the outer diameterof the helix by only enough distance to permit free movement of theelectrons.

It will, therefore, be seen that practically all of the electronemissive coating is within the space which is shielded from radiation.The amount of heating required for a given amount of electron emissionis materially diminished. By this arrangement, I find it possible toobtain the necessary electron emission with the temperature of the strip1 exceeding the temperature of the inner surface of the cylinder 2 byless than 200 C.

Those skilled in the art will recognize that many variations in thedetails of this tube and of its method of manufacture, can be madewithout departing from the spirit of my invention. The omission ofspecific mention of such variations is not intended as a limitation.

I claim as my invention:

1. A cathode comprising a directly heated screw-shaped ribbon and aradiation shield surrounding said ribbon and heated by radiationtherefrom, said ribbon and the inner surface of said shield being coatedwith electron-emissive material and the outer surface of said shieldbeing bright.

2. A cathode comprising a screw-shaped ribbon, the ribbon being edgewiseto the axis of the screw and a radiation shield surrounding said ribbon.

3. A cathode comprising a screw-shaped ribbon, the ribbon being edgewiseto the axis of the screw and a radiation shield surrounding said ribbon,said ribbon and the inside surface of said shield being coated withelectron-emissive material.

4. A cathode comprising a screw-shaped ribbon, the ribbon being edgewiseto the axis of the screw and a cylindrical radiation shield surroundingsaid ribbon, said ribbon and the inside surface of said shield beingcoated with electron-emissive material and connections for heating saidribbon directly, said shield being heated by radiation from said ribbon.

5. A cathode comprising a corrugated ribbon in screw form, thecorrugations extending across the ribbon at an angle to the axis of thescrew.

6. A cathode comprising a corrugated ribbon in screw form, thecorrugations extending across the ribbon at an angle to the axis of thescrew and being of varying depth.

7. A cathode comprising a corrugated ribbon in screw form, thecorrugations extending across the ribbon at an angle to the axis of thescrew and being deeper at the inner edge of the ribbon than at the outeredge.

8. A cathode comprising a helical portion and a cylindrical portionsurrounding the same, the equivalent radiation surface of the helicalportion being approximately equal to the area of said cylindricalportion.

9 A cathode for electrical discharge devices comprising a corrugated,edgewise-wound, helical ribbon.

10. The method of forming a screw-shaped cathode which comprisescorrugating a strip of cathode material with corrugation transverse tothe strip, winding said corrugated strip edgewise into an approximatelyhelical form-and so distorting said corrugations during said windingthat they are progressively more shallow from the inner to the outeredges of the strip.

11. The method for forming a screw-shaped cathode which comprisescorrugating a strip of cathode material with corrugation transverse tothe strip, winding said corrugated strip edgewise into an approximatelyhelical form, so distorting said corrugation during said winding thatthey are progressively more shallow from the inner to the outer edges ofthe strip and coating the wound strip with an electron-emissivematerial.

12. A cathode comprising a corrugated ribbon in screw form, thecorrugations extending across the ribbon at an angle to the axisof thescrew and at an oblique angle to the edges of the ribbon.

ERWIN F. LOWRY.

